1. Field of the Invention
The present invention relates to the field of laser optics, particularly to a mirror laser beam delivery systems used, e.g., in laser surgery, especially with a surgical endoscope. The invention also relates to a method for controlling alignment of such systems.
2. Description of Prior Art
At the present time laser techniques find increasing medical applications, in particular in laser surgery. For carrying out an operation with the use of a laser, a laser beam should be delivered to the operation site, but such sites usually are located remotely from the source of laser energy and are very often poorly accessible.
Optical systems used for delivering laser energy to the operation or treatment site are known as laser beam delivery systems. Usually such systems are built into an endoscope, which is an instrument used for visually examining the interior of body cavities.
Typically, a laser apparatus for surgical applications employing an endoscope consists of two laser beam systems which operate on different wavelengths. One laser beam system is used to generate a visible, low-power beam which indicates the target area. The second laser beam system, which is aligned with the first one, is used to provide a high-power beam which operates on an invisible wavelength. The surgeon using the apparatus guides the low-power visible beam via the endoscope to the selected site in the operation area by observing its position and then activates the power or surgical beam so that it will impinge (via the endoscope) onto such site. The visible beam has lower energy than the level required for treating an object, while the power beam has a level of energy capable of burning or ablating tissue and thus performing the operation. One typical system of this type is described in U.S. Pat. No. 4,917,083 to J. Harrington and M. Clancy, 1990.
At the present time all laser beam focusing systems (which operate through an endoscope or without an endoscope) use optical lenses as their main focusing elements. Such a focusing system, which is known as a telescope, usually consists of a tubular housing, which contains a number of lenses arranged on the general optical path and intended for focusing both laser beams on the operation area. One such system is sold under the mark Micromanipulator Model 5000 by Coherent, Inc., Palo Alto, Calif. In operation, the user, depending on the type of the procedure, must select a proper distance between the target and the outlet end of the focusing unit to insure either focusing or defocusing conditions, depending on the type of the operation.
It is known, however, that all lens type systems have refraction indexes dependent upon the wavelength of the laser beam used in the system. This causes problems which in practice cannot be solved completely. More specifically, if the power or invisible beam is in the far infrared wavelength range, i.e., 10.6 microns, and the guide, or visible beam has a wavelength of 0.632 micron, both beams will be refracted differently by the optical system. This is known as chromatic aberration.
Thus, any laser beam delivery system based on the use of optical lenses is unequivocally dedicated only to one predetermined power beam laser source wavelength. This means that each time the user wants to change the laser wavelength, for example, for changing type of a surgical procedure, such user has to replace the laser beam delivery system. All of the above will not allow the surgeon to switch from one type of the laser source to another without changing the original setup. Thus, the surgeon must purchase an additional optical delivery system (at substantial cost) for a different specific wavelength.
Aberration is a failure of a lens system to produce point-to-point correspondence between an object and its image. Chromatic aberration occurs in optical systems which act upon light of different wavelengths. In other words, two coaxial laser beams of different wavelengths incident on the same point are refracted by the system to different degrees and thus cannot be focused exactly upon the same target point by a single system.
It is also well known from optics that lights of different wavelengths, and therefore laser beams of different wavelengths, are absorbed differently; this can lead to substantial energy losses and inaccuracy. This means that existing apparatus of the optical type are not sufficiently accurate and therefore, unreliable and unsuitable for critical surgical procedures. A surgeon thus cannot be absolutely confident that both beams will be coincident on the same point and with the same spot size.
The problems of laser optical systems described above have been partially solved by applicants with the use of a method and apparatus for transmitting and steering laser beams described in their earlier U.S. patent application Ser. No. 07/576,790, filed Sept. 4, 1990. The method and apparatus disclosed in the above application are based upon the use of a two-mirror delivery system for the delivery of laser beams to the operation site. Since a mirror system does not produce chromatic aberration, the problem of inconsistency between laser sources (with fixed wavelengths) and laser beam delivery system was solved.
However, the two-mirror laser beam delivery system did not solve a problem of misalignment which may occur when the two coaxial beams were presumably focused at one point. This occurred when the visible beam was deflected from the desired position, e.g., due to misalignment, the surgeon will see the absolute position of the visible beam, but not its deflection relative to the optical axis. This is because the optical axis is an imaginary axis and cannot be physically seen. Such conditions prevented the surgeon from being confident that both beams were precisely aligned.
In addition, two-mirror systems of the type described above introduce a phenomenon which is known as central obscuration, i.e., blocking of a portion of a light beam by a smaller mirror which interferes with the path of the laser beam being delivered.
Everything described above relates to laser beam delivery systems in general. However, in addition to the problems concerning laser beam delivery systems, their incorporation into an endoscope also results in specific problems. As is known, current endoscopic laser surgery is performed with one of two types of endoscope attachments, i.e., a waveguide device with reflecting internal surfaces, and a focusing endoscope that passes the laser beam intact through an internal tube.
Since waveguide devices are characterized by diversion of the beam at the output end of the endoscope, they are applicable only when the output end of the endoscope is in direct contact with the tissue to be treated.
As far as the focusing endoscope is concerned, it allows a certain short distance between the end of the endoscope and the operation area. However, such an endoscopic tube cannot pass the entire laser beam which enters the tube without clipping a substantial portion of its energy. Otherwise it would be necessary to employ an extremely expensive and complicated optical system which would make the endoscopic system economically unjustifiable.
Furthermore, conventional systems described above have a "rigid", i.e., non-adjustable structure which does not allow elimination of slight misaligment which may occur in the system.
For the reasons described above, conventional endoscopic laser beam delivery systems have significant problems, such as large spot sizes, poor alignment of the aiming and surgical laser beams, and large transmission losses.